PostgreSQL supports a
powerful rule system for the
specification of views and ambiguous
view updates. Originally the
PostgreSQL rule system consisted
of two implementations:

The first one worked using row
level processing and was implemented deep in the
executor. The rule system was called
whenever an individual row had been accessed. This
implementation was removed in 1995 when the last official
release of the Berkeley
Postgres project was transformed into Postgres95.

The second implementation of the rule system is a
technique called query rewriting.
The rewrite system is a module that
exists between the parser stage and
the planner/optimizer. This
technique is still implemented.

The query rewriter is discussed in some detail in Chapter 37, so there is no need to cover it
here. We will only point out that both the input and the output
of the rewriter are query trees, that is, there is no change in
the representation or level of semantic detail in the trees.
Rewriting can be thought of as a form of macro expansion.